1. Field of the Invention
The invention relates to a method for the production of organosilicon compounds comprising carboxy radicals. 2. Description of the Related Art
Organosilicon compounds comprising carboxy groups are used widely as textile finishing agents. For fabrics treated in this way, good soft-hand effects are achieved, coupled with a low yellowing tendency. In addition, organopolysiloxanes comprising carboxy groups are used in the finishing of leather and as release agents.
Various methods are known for producing carboxy-functional silanes or siloxanes. The insertion of the carboxylic acid groups via Grignard methods

U.S. Pat. No. 2,763,675), from carboxyl compounds with activated CH group (reaction of haloalkylsiloxanes with cyanoacetic alkyl esters or malonic esters and subsequent hydrolysis including decarboxylation; cf. U.S. Pat. No. 3,391,177), and the (co)hydrolysis of propionyl chloride-functional silanes (obtainable by chlorinating hydrosilylation of α,β-unsaturated carboxylic esters with halosilanes; cf. U.S. Pat. No. 3,143,524) are, however, less suited as industrial methods merely on the grounds of cost. Furthermore, the proposed processes place high requirements on the purity of the starting materials and the apparatus side of process control.
According to U.S. Pat. Nos. 2,900,363, 2,957,899 and 2,875,177, organosilicon compounds comprising carboxy groups can be obtained by hydrosilylation of acrylonitrile and subsequent, acid- or base-catalyzed hydrolysis. Disadvantages of these methods are the use of the toxicologically unacceptable acrylonitrile, the price of other suitable, unsaturated, nitrile-functional starting materials, and the hydrolysis being very slow on account of the heterogeneous reaction system, as a result of which a quantitative yield of free carboxylic acid groups can only be realized at very high cost.
Furthermore, organopolysiloxanes comprising carboxy groups are obtained by reacting SiH-functional compounds with unsaturated carboxylic acids in the presence of known hydrosilylation catalysts. The selectivity of the reaction, however, is very low since a significant secondary reaction which takes place is the condensation of SiH with the acidic proton of the acid group, which proceeds with the elimination of hydrogen, meaning that hydrolysis-labile, Si—O—C-linked structures are formed to a considerable degree.
It has therefore been proposed to replace the active hydrogen of the unsaturated carboxylic acid either with alkyl radicals (EP 569189 A) or a silyl radical (EP 196169 B1; U.S. Pat. No. 4,990,643) in order, following the addition reaction of said derivatives onto SiH-containing organosilicon compounds, to hydrolytically release the carboxylic acid group again. However, the methods are likewise very expensive since the hydrolysis of the silyl protective group and in particular of the alkyl esters requires, on account of the heterogeneous system, large amounts of water, long reaction times, high temperatures and—in the case of the alkyl esters—additionally strong acids and bases as catalyst, which in turn can lead to undesired secondary reactions on the siloxane backbone. Moreover, the water used in excess has to be removed again from the heterogeneous system when the reaction is complete, which is only possible through distillation with the help of an entrainer such as toluene since otherwise the mixture foams to a considerably high degree.
According to more recent patent specifications, silanes and siloxanes comprising carboxy groups can be obtained by adding a tertiary butyl ester with olefinic double bond, such as, for example, t-butyl methacrylate or t butyl undecenoate, onto an SiH-containing organosilicon compound in the presence of a hydrosilylation catalyst, and then converting the tertiary butyl ester group into the corresponding carboxylic acid group with thermal or catalytic cleavage of gaseous isobutene (U.S. Pat. Nos. 5,504,233, 5,637,746). The greatest disadvantage of this method is that the cleavage reaction proceeds smoothly only at elevated temperature, and in the process large volumes of a highly flammable, combustible gas with an extremely low flash point are generated. In particular, the explosive gas mixtures formed with air represent a considerable danger potential. The process can thus only be realized at high cost and requires special apparatus precautions and know-how. In addition, during the catalytic cleavage of the tert-butyl ester, strong acids, such as p-toluenesulfonic acid, methanesulfonic acid or trifluoromethanesulfonic acid, are used in amounts which can adversely affect the reaction, since these acids are also typical equilibration catalysts.